This invention relates to an aqueous electroplating solution for high speed electroplating satin-bright tin-bismuth solder coatings.
Electroplated solder coatings are used extensively in the electronics industry in the manufacture of printed wiring boards (PWBs), electrical contacts and connectors, semiconductor packaging, electrical conduits and other related parts. These plated solder coatings must be pore free, corrosion resistant, and they must display long-term solderability and be free from solderability failures such as nonwetting and dewetting.
Until recently, lead and tin-lead alloys have been the most widely used solder coatings. But because of the toxicity of lead, there has been a world-wide drive to replace lead solders in electronic parts. As a consequence there is an on-going effort to find a replacement for tin-lead solder that can be xe2x80x9cdroppedxe2x80x9d into current applications. Electrodeposited tin-bismuth alloys are possible alternatives to commonly used tin-lead coatings.
Although electroplating of tin-bismuth alloys has been known for many years, it was traditionally considered to be a difficult process, and commercially viable plating baths for these alloys are yet to be developed. The major difficulty in plating tin-bismuth alloys arises from the large difference in standard deposition potential of tin and bismuth ions. This problem was traditionally solved by adding a complexing agent to change the electrodeposition reaction from simple metal ion reduction to reduction of a metal complex. The shift in the reduction potential provided by the complexing agent allows the deposition of two different metals to take place at the same potential over the wide range of current densities. Examples of these complexing agents are polydentate ligands such as EDTA, DTPA, polyamines and gluconates. Complexing usually allows very good control of the plating process and properties of the deposit. However, most of the known complexing agents are environmentally undesirable, and some of them have been banned in many countries.
A different approach to deposition of tin alloys involves utilization of a metal-specific surfactant, which can significantly slow or even inhibit deposition of one of the alloying elements in the certain potential range. The resulting formulation will be environmentally safe, provide good control of alloy composition and have a wide operating window. So far all the attempts to develop such chemistries have failed because of the strong immersion deposition of bismuth on tin parts which always takes place during the electrodeposition process. The phenomenon of immersion deposition is caused by a large difference in the deposition potentials of bismuth and tin when no complexing agent is present. As a result the more noble metal (bismuth) is deposited on the surface of the less noble metal (tin) before, during and after electroplating.
Accordingly there is a need for a new electroplating solution that can produce high speed, satin-bright plating of tin-bismuth alloy that will meet the solderability requirements for PWB and electronics packaging applications. The solution should not contain complexing agents and should avoid bismuth immersion deposition.
In accordance with the present invention, an electroplating solution is provided for electroplating satin bright tin-bismuth alloy solder coatings in high speed electroplating applications. The solution comprises a sulfonic acid electrolyte, a soluble tin compound, a soluble bismuth compound, a non-ionic surfactant, a grain refiner and an antioxidant. The preferred non-ionic surfactant comprises a mixture of polyethylene glycol-block-polypropylene glycol, polyethylene glycol-ran-polypropylene glycol, and ethylenediamine tetrakis (polyethylene glycol-block-polypropylene glycol) tetrol.